1. Field of the Invention
Embodiments of the invention relate to the field of asphalt concrete pavement production and include slurries of water and zeolite. These slurries have beneficial properties when mixed with asphalt cement. Methods of making and using the slurries, as well as asphalt cement mixtures incorporating the slurries are also included herein.
2. Description of the Related Art
Asphalt cement, also known as bitumen, mastic, or asphalt binder, is made up primarily of high molecular weight aliphatic hydrocarbon compounds, but also small concentrations of other materials such as sulfur, nitrogen, and polycyclic hydrocarbons (aromatic and/or naphthenic) of very low chemical reactivity. Asphalt cement is a combination of asphaltenes and maltenes. Maltenes are typically present as resins and oils. Asphaltenes are more viscous than either resins or oils and play a major role in determining asphalt viscosity. Oxidation of aged asphalt causes the oils to convert to resins and the resins to convert to asphaltenes, resulting in age hardening and a higher viscosity binder. In U.S. and Polish terminology, asphalt (or asphalt cement) is the carefully refined residue from the distillation process of selected crude oils. Outside these countries, the product is often called bitumen.
The largest use of asphalt cement is for making asphalt concrete pavement for road surfaces. This accounts for approximately 85% of the asphalt consumed in the United States. Asphalt concrete pavement material is commonly composed of 5 percent asphalt cement and 95 percent aggregates (stone, sand, and gravel). Due to its highly viscous nature, asphalt cement must be heated so that it can be mixed with the aggregates at the asphalt mixing plant. For simplification of the terminology asphalt concrete pavement will be denominated “asphalt mix” from here.
One drawback to using asphalt mix is the high energy cost associated with reaching temperatures that improve handling and placement on road surfaces. Typically, asphalt mix may need to be produced at temperature as high as 160° C. for effective paving. Paving operation includes the storage of asphalt mix in the plant silos, the transport of the asphalt mix to the job site, the handling of the asphalt mix out of the trucks and into the paving equipment, the placement of the asphalt mix in the road surface with adequate compaction and specified densities. During all this process the asphalt mix needs to keep good workability thus requiring heating at higher temperatures. For special asphalt mixes like rubber asphalt higher temperatures of 170 to 180° C. are required.
There are some options available to allow a reduction in the production temperatures and they are known as Warm Mix Asphalt (WMA). Warm Mix Asphalt is the generic term for a variety of technologies that allow the producers of asphalt mix pavement materials to lower the temperatures at which the material is produced and placed on the road, without compromising the workability required to execute the paving job.
There are a number of technologies available today for Warm Mix Asphalt and generically the technologies can be divided into three categories: chemical additives, synthetic zeolites, and water foaming mechanical systems.
Chemical additives including Fisher Tropsch wax or chemical packages that may or may not contain emulsion technology can be employed to allow a reduction in temperatures. These may reduce asphalt mix production temperature to as low as 110° C. These chemical additives either change asphalt cement properties or allow a better dispersion of asphalt cement into the mix.
SASOBIT®, a product of Sasol International, is one well-known additive. It is a Fisher Tropsch wax with a longer chain than a typical paraffin wax. It can be pre-blended with the asphalt cement at the terminal, or added as small beads at the asphalt mix plant. The wax works by reducing the overall viscosity of the mix. It may cause an increase in the PG grade of the mix, a factor which may have to be taken into consideration in the mix design. The term PG grade stands for “performance grading” and is reported using two numbers—the first being the expected average seven-day maximum pavement temperature (° C.) and the second being the expected minimum pavement design temperature likely to be experienced (° C.). Thus, a PG 58-22 is intended for use where the expected average seven-day maximum pavement temperature is 58° C. and the expected minimum pavement temperature is −22° C. Notice that these numbers are pavement temperatures and not air temperatures.
Another well-known chemical additive is EVOTHERM®, a product of MeadWestvaco Asphalt Innovations. It is an emulsion technology, in varying forms, which needs a customized chemical package for each type of mix. It can be pre-blended with the asphalt cement at the terminal, or added as liquid at the asphalt mix plant, mixing with the asphalt cement.
Synthetic zeolites are able to promote time released micro bubbles foaming when added to the asphalt mix. ADVERA® WMA, a product from PQ Corporation, is a hydrated zeolite sodium A powder composed of around 78% zeolite and 22% water. Zeolite is used in asphalt production as a water release agent to induce a controlled and efficient foaming process that aids in the workability of the asphalt mix and allows effective coating of the aggregate to take place at lower production temperatures. Like chemical additives and chemical packages, the addition of synthetic zeolites like ADVERA® WMA also allows a reduction in the asphalt mix production temperature to as low as 110° C.
Water foaming mechanical systems is a technology promoted mostly by the asphalt industry equipment suppliers (including, for example Astec, Gencor, Terex, Maxam, Meeker, Stansteel, Reliable). The water is injected into the asphalt cement feed line to the asphalt mix production drum and upon contacting the hot asphalt cement all water converts to steam bubbles with approximately 20 mesh to ⅛″ diameter which increased volume by a factor of 18 times. There will be a significant volumetric increase of the asphalt cement at this stage. Because the large bubbles travel fast through the asphalt mix the workability improvement does not stay for a longer length of time so the reduction of asphalt production temperatures are not as low as the ones observed with the chemical additives, chemical packages or synthetic zeolites. Typically the water foaming technologies allow a reduction of asphalt mix production temperatures to as low as 135° C.
Typically the chemicals additives and chemical packages are added on a weigh percent rate of asphalt cement varying from 0.5% up to 7%. The rate of addition depends on the type of chemical additive or chemical package being used. In general terms the chemical additives or chemical packages add an extra $2 to $4 per ton of asphalt mix in variable cost. The reduction of production temperatures to 110° C. allow savings on energy (fuel) required to dry the aggregates of about $0.4 to $0.5 per ton of asphalt, depending on the type and cost of fuel being used.
In some embodiments synthetic zeolites are added on a weigh percent rate of asphalt mix varying from 1.3 up to 2.8 Kg per Ton of Mix. In general terms the synthetic zeolite add an extra $1 to $2 per ton of asphalt mix in variable cost. The reduction of production temperatures to 110° C. allow savings on energy (fuel) required to dry the aggregates of about $0.4 to $0.5 per ton of asphalt, depending on the type and cost of fuel being used.
Typically foaming water is added on a weigh percent rate of asphalt cement varying from 1% up to 3%. Water does not add any extra cost per ton of asphalt mix in variable cost. The reduction of production temperatures to 135° C. allow a saving on energy (fuel) required to dry the aggregates of about $0.2 to $0.3 per ton of asphalt, depending on the type and cost of fuel being used.
The extra variable cost of the chemical additives, chemical packages or synthetic zeolites can be offset by the improvements on the asphalt mix. One major notorious improvement is in the workability of the mix allowing the increase of RAP (reclaimed asphalt pavement) or RAS (recycled asphalt shingles) into Warm Mix Asphalt mixes when compared to Hot Mix Asphalt Mixes. Because RAP and RAS carries bitumen on their composition an increase in weight percentage of their use into the asphalt mix will decrease the amount of fresh asphalt cement used, with considerable saving on variable cost. Typically for every 10% increase in the use of RAP the variable cost benefit is in the order of $2 to $3 per ton of asphalt mix. For every 1% increase in the use of RAS the variable cost benefit is in the order of $1.0 to $1.5 per ton of asphalt mix.
Water foaming mechanical systems raise some considerations that may complicate their use:                Brownian Motion laws of physics show that the large bubbles may travel rapidly out of the mix;        If sand or other fines are used in the mix, they can potentially act as an anti-foam and eventually break more of the remaining bubbles;        Foaming (bubbles) provide the improved workability of the mix but it could possibly decline as the bubbles travel out of the mix;        Foaming (bubbles) are not time released therefore with long hauls the workability of the asphalt mix may be reduced as bubbles coalesce and travel out of the mix;        May limit low end asphalt mix production temperature when compared to chemical additives, chemical packages or synthetic zeolites;        May not allow as much an increase of RAP or RAS compared to the chemical additives, chemical packages or synthetic zeolites due to increased stiffness of the mix;        
Zeolites are microporous crystalline solids with well-defined structures. Generally they contain silicon, aluminum and oxygen in their framework and cations (such as Na+, K+, Ca2+, Mg2+ and others), water and/or other molecules within their pores. These positive ions are rather loosely held and can readily be exchanged for others in a contact solution. Many occur naturally as minerals, and are extensively mined in many parts of the world. Others are synthetic, and are made commercially for specific uses. An example mineral formula is: Na2Al2Si3O10-2H2O, the formula for natrolite. Naturally-occurring mineral zeolites include amicite, analcime, barrerite, bellbergite, bikitaite, boggsite, brewsterite, chabazite, clinoptilolite, cowlesite, dachiardite, edingtonite, epistilbite, erionite, faujasite, ferrierite, garronite, gismondine, gmelinite, gobbinsite, gonnardite, goosecreekite, harmotome, herschelite, heulandite, laumontite, levyne, maricopaite, mazzite, merlinoite, mesolite, montesommaite, mordenite, natrolite, offretite, paranatrolite, paulingite, pentasil (also known as zsm-5), perlialite, phillipsite, pollucite, scolecite, sodium dachiardite, stellerite, stilbite, tetranatrolite, thomsonite, tschernichite, wairakite, wellsite, willhendersonite, and yugawaralite.
There are several types of synthetic zeolites that form by a process of slow crystallization of a silica-alumina gel in the presence of alkalis and organic templates. The product properties depend on reaction mixture composition, pH of the system, operating temperature, pre-reaction ‘seeding’ time, reaction time as well as the templates used. Preparation of synthetic zeolites suitable for use in embodiments of the invention is shown, for example, in U.S. Pat. No. 4,661,334, to Latounnette, et al. (“Preparation of Zeolites 4A and/or 13X”); U.S. Pat. No. 4,649,036 to Pastorello, et al. (“Process for the Manufacture of Zeolites 4A . . . ”); U.S. Pat. No. 5,487,882 to Hu, et al. (“Process for Preparation of Zeolite ‘X’”); U.S. Pat. No. 6,258,768, to Araya (“Zeolite P . . . ”); and U.S. Pat. No. 4,264,562, to Kostinko (“Method of Producing Zeolite Y”).
Synthetic zeolites hold some key advantages over their natural analogs. The synthetics can, of course, be manufactured in a uniform, phase-pure state. It is also possible to manufacture desirable zeolite structures which do not appear in nature. Zeolite A is a well-known example. Examples of synthetic zeolites are the A, P, X and/or Y types. One example of a type A zeolite has the chemical formula Na2O:2SiO2:Al2O3:3.94H2O, wherein the quantity of Na2O is 17%, Al2O3 is 29%, SiO2 is 34% and H2O is 20%. U.S. Pat. No. 4,264,562, to Kostinko gives a description of different synthetic zeolite types.
The general formula for zeolites can be expressed by Na2O:χSiO2:Al2O3:γH2O. Zeolite X will have χ=2.5±0.5, Zeolite A will have χ=1.85±0.5, Zeolite Y will have χ=4.5±1.5. U.S. Pat. No. 6,258,768 (Arraya) describes the typical formula for Zeolite P where χ will vary from 1.80 up to 2.66. The water content on the structure, represented by γ is variable and can reach up to 9. Typical values of γ for Zeolite X are 6.2 and Zeolite A is 3.91. In some embodiments the value of γ is in a range of 3 to 9 for synthetic grades. Natural grades are typically 2. For example, natrolite has the formula of Na2Al2Si3O10.2H2O. One skilled in the art will recognize that the different water retention for the different zeolites will affect the amount of zeolite that is useful in processes according to the invention.